Temperature measurement and control for laser and light-emitting diodes

ABSTRACT

The existing diodes in an LED or laser diode package are used to measure the junction temperature of the LED or laser diode. The light or laser emissions of a diode are switched off by removing the operational drive current applied to the diode package. A reference current, which can be lower the operational drive current, is applied to the diode package. The resulting forward voltage of the diode is measured using a voltage measurement circuit. Using the inherent current-voltage-temperature relationship of the diode, the actual junction temperature of the diode can be determined. The resulting forward voltage can be used in a feedback loop to provide temperature regulation of the diode package, with or without determining the actual junction temperature. The measured forward voltage of a photodiode or the emissions diode in a diode package can be used to determine the junction temperature of the emissions diode.

BACKGROUND

Semiconductor light-emitting devices such as light-emitting diodes(LEDs) and laser emitting devices such as laser diodes are used in anincreasing number of applications. LEDs, for instance, are widely usedin general illumination applications while laser diodes are commonlyused in long-distance communication applications, optical applicationsand various imaging applications. As the use of these devices hasincreased, so too have various application demands for more preciseoperation of the devices. LEDs need to meet particular brightness,stable light point and ratio of light output requirements, and laserdiodes need to meet particular lasing wavelength requirements, bothwhile maintaining adequate operational lives.

Among the operational requirements of both LEDs and laser diodes is aminimum degree of temperature control. In LEDs, light output varies withthe junction temperature of the LED, affecting intensity and causingspectral shift. In laser diodes, the emission wavelength varies with thejunction temperature of the laser diode. Generally, the power deliveredto the LED or laser diode package is controlled by a feedback loop thatincludes an estimation of the temperature of the package. One or morethermal sensors are often attached to the package to estimate theoperating temperature. A thermistor or thermocouple may be used, forexample. Among the deficiencies of such a thermal sensor is its lack ofproximity to the semiconductor die in which the diode is formed, whichcan result in errors when estimating junction temperature. This lack ofprecision may not be acceptable in many of today's applications for LEDsand laser diodes.

SUMMARY

The existing and inherent diodes in a light-emitting diode or laserdiode package are used to measure the junction temperature of the LED orlaser diode, hereinafter referred to generally as emission diodes. Thelight or laser emissions of the diode are switched off by removing theoperational drive current applied to the diode package. A referencecurrent, which can be lower than the operational drive current, isapplied to the diode package. The resulting forward voltage of theemissions diode is measured using a voltage measurement circuit. Usingthe inherent current-voltage-temperature relationship of the emissionsdiode, the actual junction temperature of the emissions diode can bedetermined. The resulting forward voltage can be used in a feedback loopto provide temperature regulation of the diode package, with or withoutdetermining the actual junction temperature.

In packages including a monitor photodiode for measuring the emissionsof the LED or laser diode, the reference current can be applied to thephotodiode in place of or in addition to the emission diode, followed bymeasuring the forward voltage across the photodiode. The measuredtemperature of the photodiode can be used in place of or in combinationwith the measured temperature of the emission diode to determine theemission diode junction temperature.

One embodiment includes operating an emissions diode for junctiontemperature determination. A drive current is applied to the emissionsdiode at a level sufficient to enable emissions by the emissions diode.After driving the emissions diode, the drive current is removed and areference current is applied to the emissions diode at a level less thanthe drive current level. With the reference current applied, the forwardvoltage level of the emissions diode is measured. Using the measuredforward voltage level, an operating temperature of the emissions diodeis controlled. Controlling the operating temperature can includedetermining a junction temperature of the emissions diode based on themeasured forward voltage level and the reference current level.

One embodiment includes driver circuitry for an emissions diode. Thedriver circuitry includes at least one current source, at least onemeasurement circuit and at least one control circuit. The current sourcesupplies a drive current for the emissions diode and a reference currentfor a photodiode in a diode package with the emissions diode. Thevoltage measurement circuit measures a forward voltage of the photodiodewhile the reference current is applied to the photodiode. The controlcircuit regulates an operating temperature of the diode package based onthe measured forward voltage of the photodiode.

The temperature measurement and control can be used in manyapplications, such as for creating depth images in systems utilizinglaser diode light sources. Accordingly, one embodiment includes drivinga light source including a laser diode in a diode package having aphotodiode by applying a drive current to the laser diode to causeemissions at a predetermined wavelength. One or more objects areilluminated with the light source at the predetermined wavelength. Oneor more depth images including the one or more objects are sensed orcreated while illuminating the one or more objects. A reference currentis applied to the diode package followed by measuring a forward voltageacross the laser diode and/or the photodiode. The temperature of thediode package is controlled based on the forward voltage across thelaser diode and/or the photodiode to maintain emissions by the laserdiode at the predetermined wavelength.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter. Furthermore, the claimed subject matter is not limited toimplementations that solve any or all disadvantages noted in any part ofthis disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional laser diode package in aperspective view.

FIG. 2 is a block diagram of driver circuitry in accordance with oneembodiment for driving an emissions diode package.

FIG. 3 is a flowchart describing a method of determining the junctiontemperature of an LED or laser diode in accordance with one embodiment.

FIG. 4 is a flowchart depicting one embodiment for correlating ameasured forward voltage of an emissions diode to the junctiontemperature of the emissions diode.

FIG. 5 is a flowchart depicting one embodiment of correlating themeasured forward voltage of a diode to junction temperature withoutactually calculating the junction temperature.

FIG. 6 is a flowchart depicting a method of performing temperaturecontrol in accordance with one embodiment.

FIG. 7 is a flowchart depicting a method of characterizing and settingdriver currents for multiple diode packages.

FIG. 8 illustrates a tracking system with a user playing a game.

FIG. 9 illustrates a tracking system with a user playing a game.

FIG. 10 illustrates a capture device that may be used as part of atracking system.

DETAILED DESCRIPTION

The junction temperature of a laser-emitting diode (LED) or laser diodeis determined using a measured forward voltage across one or moreexisting diodes in the package housing the emissions diode. The LED orlaser diode itself is driven with a low-level reference current, and theresulting voltage across the emissions diode is determined. Based on themeasured voltage, the temperature across the junction of the emissionsdiode is calculated. The calculated temperature can be used in afeedback loop to maintain the emissions diode at a desired temperatureby heating or cooling the diode package.

In one example, a method of driving a laser diode is provided thatincludes driving the laser diode with an operational current to causeemission by the laser diode. The laser diode emissions are paused byswitching in a low-level reference current to the laser diode, followedby measuring the forward voltage across the laser diode. Using theinherent current-voltage-temperature relationship of a PN junction, thejunction temperature of the laser diode is determined. After switchingin the low-level reference current and/or measuring the resultingforward voltage, the laser diode is again driven with the operationalcurrent to cause emissions by the laser diode.

In one example, the reference current is additionally applied to amonitor photodiode within the laser diode package. The resulting voltageacross the photodiode is measured, followed by determining the junctiontemperature of the photodiode using the current-voltage-temperaturerelationship of the PN junction. An offset is applied in one embodiment,to compensate for any difference in temperature between the monitorphotodiode diode and the laser diode die. In another example, themeasured voltage across the laser diode and the measured voltage acrossthe monitor photodiode are combined and used to determine the junctiontemperature of the laser diode. The above processes may be used for LEDsas well as laser diodes.

In one example, the monitor photodiode alone is used to determine theLED or laser diode junction temperature. The operational drive currentcan be applied to the emissions photodiode while applying the low-levelreference current to the photodiode. Using the measured forward voltageacross the photodiode, the junction temperature of the emission diode isdetermined.

FIG. 1 is a schematic diagram showing a typical laser diode package in aperspective view. The laser diode package 102 includes a case 104 (alsocalled a can) which houses a die for laser diode 106, a die for monitorphotodiode 108 and a heat sink 110. Heat sink 110 is used to dissipateheat generated by the laser diode die. The case includes a window 112through which the laser beam generated by the laser diode die passes.The laser beam is represented by arrow 114 in FIG. 1. Generally, laserdiodes will emit power from both ends of the resonant cavity in whichthe diode structure is contained. The beam or emissions from the rearfacet of the laser diode in FIG. 1 is represented by arrow 116. The rearemission from laser diode 106 is monitored by a monitor photodiode 108.The photodiode utilizes optical charge carrier generation to sense andmeasure the light produced by the laser diode. By monitoring the outputof the laser diode, the power output of the laser diode can bemaintained at a constant power level using feedback control provided byan external diode driver circuit. It is noted that embodiments inaccordance with the present disclosure can be used with laser diodepackages that do not include a monitor photodiode as well as those thatdo. Furthermore, the present disclosure is equally applicable tolight-emitting diodes, which share a similar configuration to thatdepicted in FIG. 1.

FIG. 2 is a block diagram of a circuit 200 for driving a laser diodepackage 102 in accordance with one embodiment. Laser diode 106 andmonitor photodiode 108 are mounted in package 102 with leads PD, COM andLD connected to laser diode driver circuitry 200. Although thecomponents in FIG. 2 are depicted as a single laser diode driver circuit200, they can be incorporated in numerous variations, including withintypical driver circuitry or as external components thereof. Further,although the description of FIG. 2 is presented with respect to laserdiode driving circuitry, it will be appreciated that the concepts may beapplied to light-emitting diode driving circuitry.

The laser diode lead LD is connected to a first lead of the drivercircuitry, itself connecting to adjustable current source 224, low-levelcurrent source select circuit 220 and current control system 218. Whendriving laser diode 102 to produce laser emissions, current controlsystem 218 maintains a constant current output via adjustable currentsource 224. The adjustable current source responds to the variouscontrol systems to drive current to laser diode 106 between a supplyvoltage and current source. The laser diode may be placed between thecurrent source and ground. The actual current level through the laserdiode is measured by the current control system 218 and compared with aset drive current level. The set drive current level can be representedby an analog voltage and compared with the actual current level, alsorepresented as an analog voltage. Using the difference between the twovoltages, representing an error, the output level can be adjusted.

Photodiode lead PD connects to a second lead of the driver circuitry,itself connecting to power control system 210 and a second voltagemeasurement circuit 212. During emissions of laser diode 106, themonitor photodiode 108 produces a current somewhat proportional to thelaser diode's output optical power. Because of intrinsic differences ineach laser diode package, the photodiode transfer function varieswidely. The photodiode current can be measured using an ammeter withinthe power control circuitry and used as feedback, with the power controlsystem trying to keep the photodiode current, and the laser diodeoptical power constant. The output of adjustable current source 224 mayvary to keep the optical power level the same.

During operation of the laser diode package, driver circuitry 200utilizes low-level current source select circuit 220, voltagemeasurement circuits 212 and 222, temperature controller 216 andthermoelectric cooler (TEC) 214 to regulate the temperature of package102. The low-level current source select circuit 220 steps down thecurrent supplied by current source 224 to a constant low referencelevel. The reference current level is not enough to drive laseremissions from laser diode 106. With the reference current applied tothe laser diode, the resulting forward voltage of the diode is measuredusing voltage measurement circuit 222. The voltage measurement circuitprovides the measured voltage level to the current control system. Inone embodiment, the current control system determines the junctiontemperature of the laser diode 106 using the measured voltage level. Inanother embodiment, the actual junction temperature is not calculated,but the measured forward voltage is used to correlate the junctiontemperature, such as by comparing the measured forward voltage to atarget forward voltage corresponding to the desired operatingtemperature. Based on the actual junction temperature or just themeasured forward voltage, temperature control for the diode package isperformed by temperature controller 216. In one embodiment,thermoelectric cooler 214 includes a peltier diode operating as a heatengine in which charge carriers absorb band gap energies which arereleased as heat. Although TEC 214 is shown within the driver circuitry200 in FIG. 2, it is often mechanically coupled to the case of laserdiode package 102. Other temperature regulating devices such as simplefans and resistive heaters can be used to regulate temperature. In oneembodiment, voltage measurement circuit 222 provides the voltagedirectly to the temperature controller which performs the junctiontemperature correlation in place of current control system 218.

The current-voltage-temperature relationship of a diode can be seen byexamining the ideal diode equation set forth in equation 1:I _(D) =I _(S)(e ^(V) ^(D/) ^((nV) ^(T)) −1   equation 1

I_(D) is the diode current, I_(S) is the reverse bias saturationcurrent, V_(D) is the voltage across the diode, V_(T) is the thermalvoltage and n is the emission coefficient. The thermal voltage is aknown quantity, equal to about 25.85 mV at room temperature. The thermalvoltage is defined by equation 2:V _(T)=^(kT)/_(q)   equation 2

T is the absolute temperature of the PN junction, q is the magnitude ofcharge on an electron and k is boltzmann's constant. When the currentthrough the diode is a known reference current as set forth above, thejunction of the temperature of the diode or PN junction can be set forthas in equation 3, where Z is a constant representing the ratio of diodecurrent to saturation current:T=ZV _(D)   equation 3

As such, the temperature of the junction of the diode can be representedas the product of a known constant and the measured voltage across thediode. It has been shown that generally the voltage across a silicondiode will vary at a rate of approximately −2.2 mV/° C., which can beused in equation 3 for the value of Z.

Accordingly, the junction temperature of the laser diode die can bedetermined by examining the voltage across the diode at a knownreference current. In one embodiment the junction temperature isdetermined using lookup table 223 and the measured voltage level acrossthe laser diode. The lookup table contains junction temperature valuescorresponding to various measured voltage levels. Using the measuredvoltage level as an input, the lookup table returns a correlatedtemperature value.

In another embodiment, the current control system does not determine theactual junction temperature, but compares the measured voltage level toa reference voltage level representing a target junction temperature.The reference voltage level can include characterizations of the deviceto correlate the measured voltage to the junction temperature. Forexample, the laser diode may be characterized by measuring the forwardvoltage under test conditions at various known temperatures to determinethe forward voltage corresponding to the desired operating temperature.This voltage may then be used as the comparison voltage to determinewhether to heat or cool the package.

Photodiode 108 may also be used to determine the junction temperature oflaser diode 106. In FIG. 2, the photodiode 108 is connected to a secondvoltage measuring circuit 212. In one embodiment, a single voltagemeasurement circuit is used in place of voltage measurement circuit 212and voltage measurement circuit 222. Low-level current source selectcircuit can switch in the low-level reference current to be appliedacross the photodiode. With the known low-level reference currentapplied, measurement circuit 212 determines the resulting forwardvoltage. The voltage measurement circuit provides the measured voltagelevel to power control system 210, which will correlate the measurevoltage level to the junction temperature of the laser diode. In oneexample, the power control system utilizes lookup table 226 to determinethe laser diode junction temperature based on the measured voltage ofthe photodiode. Because the photodiode typically is formed on a separatedie from the laser diode, the lookup table may incorporate an offset tocompensate for variations between the die. During characterization ofthe diode package, the lookup table can be propagated by correlatingmeasured voltages of the photodiode to known temperatures of the laserdiode to determine the corresponding values. As with the laser diode,the control system may not determine the actual junction temperature inone embodiment, but instead use the measured voltage directly in afeedback loop to regulate the diode package temperature. The measuredvoltage can be compared with a known voltage corresponding to thedesired temperature to regulate the package temperature. Voltagemanagement circuit 212 can provide the measured voltage to temperaturecontroller 216 in one embodiment, which will perform the junctiontemperature correlation in place of power control system 210.

In one embodiment, both the photodiode 108 and laser diode 106 can beused to determine the junction temperature of the laser diode. Forexample, the junction temperature as determined by the photodiode can becombined with the junction temperature as determined using the laserdiode to determine the actual junction temperature of the laser diode.In another example, the measured voltage across the photodiode iscompared with a known photodiode voltage corresponding to the desiredoperating temperature of the package to determine a difference betweenthe two. The measured voltage across the laser diode can be comparedwith a known laser diode voltage corresponding to the desired operatingtemperature of the package to determine a difference between the two.The two differences can be combined to an overall voltage differencevalue. The overall voltage difference value can be used as an input intothe feedback loop to regulate the package temperature. Similarly, acombination of both measured voltages (e.g., an average) can be used asthe input to a lookup table to determine junction temperature values.

FIG. 3 is a flowchart describing a method of determining the junctiontemperature of an LED or laser diode in accordance with one embodimentof the present disclosure. At step 302, the emissions diode is drivenwith a drive current sufficient to cause laser or light emissions fromthe diode. As will be described hereinafter, the actual value of thedrive current may be unknown in some implementations, such as wherecharacterization of the device results in setting the drive current tomeet some output specification (e.g., lasing wavelength, light outputlevel) and not maintaining an indication of the drive current levelwithin the emissions diode package.

At step 304, laser or light emissions by the emissions diode areswitched off or stopped. At step 306, a low-level current source isenabled and at step 308, a low-level reference current is applied to theemissions diode and optionally, to a monitor photodiode in the diodepackage with the emissions diode. It is noted that steps 304-308 maycomprise a single operation whereby the drive current is switched offand the low-level reference current is driven on the input line(s) ofthe diode package. After applying the low-level reference current to theemissions diode and optionally the photodiode, the resulting forwardvoltage across the emissions diode and optionally the photodiode ismeasured at step 310.

At step 312, the measured forward voltage(s) is correlated to thejunction temperature of the emissions diode. In one embodiment,correlating the measured forward voltage to the junction temperatureincludes using the measured voltage and reference current in thecurrent-voltage-temperature relationship of the diode to determine thejunction temperature. The correlation may be based on the measuredforward voltage of the emissions diode and/or the measured forwardvoltage of the monitor photodiode. If both voltages are used, they maybe averaged or combined in some other predetermined relationship todetermine a voltage level to be used in looking up a correspondingjunction temperature. Step 312 may include accessing a lookup tableusing one, both or a combination of the measured voltage levels todetermine a junction temperature value. In one example, both voltagescan be used to lookup a corresponding junction temperature and then thetwo temperature values can be averaged or combined in some otherpredetermined manner to determine the junction temperature of theemissions diode. In another embodiment, the actual junction temperatureis not determined, but the measured voltage(s) is compared with areference voltage level corresponding to a known temperature quantity todetermine whether the junction temperature is above or below the knowntemperature quantity. Again, one, both or a combination of the measuredvoltage levels may be used as the comparison against the knowntemperature quantity. The two measured levels may be combined into asingle value for comparison (e.g, by averaging) or each mayindependently compared to the reference voltage level to determinewhether the junction temperature is above or below the known temperaturequantity.

At step 314, temperature control or regulation of the diode package isperformed based on correlating the forward voltage(s) to the junctiontemperature of the diode. If step 312 includes determining the actualjunction temperature, such as by accessing a lookup table, step 314 caninclude heating or cooling the diode package based on the actualjunction temperature. If the actual junction temperature is greater thanthe target operating temperature, the package can be cooled. If theactual junction temperature is less than the target operatingtemperature, the package can be heated. If step 312 does not includedetermining the actual junction temperature, but instead, comparing themeasured voltage with a predetermined reference voltage corresponding tothe target operating temperature, step 314 can include heating orcooling the diode based on this comparison. As shown above, the junctiontemperature of a diode will decrease at a rate of about 2.2 mV/° C.Accordingly, if the measured voltage is greater than the referencevoltage, the junction temperature is lower than the target operatingtemperature and the package can be heated. If the measured voltage isless than the reference voltage, the junction temperature is greaterthan the target operating temperature and the package can be cooled. Inaddition to or in place of directly heating or cooling the diodepackage, the temperature control at step 314 can include adjusting thepower applied to the emission diode from driver circuitry 200.

At step 316, the drive current is reapplied to the diode package tocause emissions by the emissions diode. Although step 316 is shown asbeing performed after the temperature control at step 314, the drivecurrent may be reapplied anytime after measuring the forward voltage(s)at step 310. At step 318, it is determined whether the time since thelast junction temperature measurement is greater than a threshold time.If the time is not greater than the threshold, the method waits withoutperforming another measurement. When the time is greater than thethreshold, the method continues at step 304 by stopping diode emissionsto prepare for the voltage measurement at the low-level referencecurrent.

FIG. 4 is a flowchart depicting one embodiment for correlating ameasured forward voltage of an emissions diode to the junctiontemperature of the emissions diode, as can be performed at step 312 ofFIG. 3. At step 402, the emissions diode forward voltage is determinedfrom the measurement taken at step 310. After determining the emissionsdiode forward voltage, a lookup table is accessed at step 404 using themeasured forward voltage as an index into the table. At step 406, thejunction temperature of the emissions diode is determined from the entrycorresponding to the measured forward voltage.

In one embodiment, the lookup table is populated with entries duringcharacterization of the diode package. Under test conditions, thevoltage across the emissions diode can be measured under knowntemperatures. The measured voltages can be used as the index for thelookup table, with the corresponding temperature as the lookup value.Extrapolation can be used to build a comprehensive table of manytemperature values without performing measurements at every temperatureto be included in the table.

While FIG. 4 describes determining the emissions diode junctiontemperature based on the measured voltage of the emissions diode,similar steps can be performed to determine the emissions diode junctiontemperature based on a measured voltage of a monitor photodiode includedin the emissions diode package. The measured forward voltage of thephotodiode can be determined, followed by accessing a lookup table usingthe measured voltage to determine a corresponding junction temperatureof the emissions diode. The lookup table for the photodiode can bedifferent than the lookup table for the emissions diode and containvalues based on a characterization that includes the photodiode. Thephotodiode forward voltage can be measured at known temperatures of theemissions diode to develop values for the index of the lookup table. Ifthe forward voltages of both the photodiode and the emissions diode aremeasured, the method of FIG. 4 can include accessing different lookuptables to determine a junction temperature based on the measured forwardvoltage of the photodiode and a junction temperature based on themeasured forward voltage of the emissions diode. The two junctiontemperatures can be combined by averaging or applying anotherpredetermined relationship to determine a final junction temperaturevalue for the emissions diode. In another example, the two measuredvoltage values can be combined, then the combined value used as an indexinto a lookup table to determine the junction temperature of theemissions diode.

In FIG. 4, the measured forward voltage of one or more diodes in thepackage is utilized to determine the junction temperature of theemissions diode. In other embodiments, the junction temperature is notexplicitly determined, and the measured forward voltage itself is usedto regulate the package temperature, as set forth in the example of FIG.5 which can be performed for step 314 of FIG. 3. At step 502, the diodeforward voltage is determined from the measurement taken at step 310. Atstep 504, the diode forward voltage is compared with a reference forwardvoltage corresponding to a target operating temperature of the diodepackage. In one embodiment, the target operating temperature and itscorresponding reference forward voltage can be determined during acharacterization of the diode package. The target operating temperaturecan be selected for the desired lasing wavelength of a laser diode or atarget light or spectral output for a light-emitting diode. With thetarget operating temperature selected, the corresponding forward voltagelevel can be determined by measuring the forward voltage underapplication of the low-level reference current at the target operatingtemperature.

At step 506, it is determined whether the measured forward voltage isequal to the reference voltage level based on the comparison at step504. If the measured forward voltage is equal to the reference voltage,it is determined at step 508 that the junction temperature of theemissions diode is equal to the target operating temperature. If themeasured forward voltage is not equal to the reference voltage level, itis determined at step 510 whether the measured forward voltage isgreater than or less than the reference voltage level. If the measuredforward voltage is greater than the reference voltage level, it isdetermine at step 512 that the junction temperature is less than thetarget operating temperature. If the measured forward voltage is lessthan the reference voltage level, it is determined at step 514 that thejunction temperature is greater than the target operating temperature.

The forward voltage utilized in FIG. 5 can be the measured forwardvoltage of the emissions diode in the diode package or a monitorphotodiode in the package. If the monitor photodiode is utilized, thereference voltage used in the comparison can be a reference forwardvoltage of the photodiode determined to correspond to the targetoperating temperature of the diode package or the emissions diode. Thereference voltage can be determined during a characterization of thediode package. In one embodiment, both the measured forward voltage ofthe emissions diode and the monitor photodiode can be used to correlatejunction temperature. In such a case, the measured forward voltage ofboth diodes is compared to a reference voltage level. The referencevoltage level can be the same or different for each diode. Eachcomparison will yield a difference (if any) between the referencevoltage level and the corresponding measured voltage level. If thecombined difference indicates a voltage greater than a combinedreference voltage corresponding to the target operating temperature, theemissions diode temperature is determined to be less than the targetoperating temperature. If the combined difference indicates a voltageless than the combined reference voltage, the junction temperature ofthe emissions diode is determined to be greater than the targetoperating temperature.

FIG. 6 is a flowchart depicting a method for performing temperaturecontrol, as may be executed at step 314 of FIG. 3. At step 602, it isdetermined whether the calculated junction temperature is equal to areference or target operating temperature for the diode package. In analternate embodiment, step 602 can include determining whether themeasured forward voltage of the emissions diode is equal to a referencevoltage level. If the calculated junction temperature is equal to thereference temperature level (or if the measured voltage equals thereference voltage), the temperature of the package is unaffected and themethod continues at step 316 by enabling the drive current source. Ifthe calculated junction temperature is not equal to the target operatingtemperature, it is determined whether the calculated temperature isgreater than the reference temperature at step 604. If the temperatureis greater than the reference, the laser diode package is cooled at step606. If the temperature is less than the reference, the package isheated at step 608. Again, the comparison at step 604 may be of themeasured forward voltage and a reference voltage rather than directtemperatures. If the measured forward voltage is greater than thereference voltage, the diode package is heated at step 608. If themeasured forward voltage is less than the reference voltage, the diodepackage is cooled at step 606. After heating or cooling the diodepackage, the method continues at step 316 of FIG. 3 by enabling thedrive current source.

As already noted, a low-level reference current is applied to theemissions diode in various embodiments to determine the junctiontemperature using the current-voltage-temperature relationship inherentin all PN junctions. FIG. 7 demonstrates a benefit of such a technique,particularly in an application where diode package lots are used tomanufacture products and the individual diode packages have differentoperating characteristics, which is typical of manufactured packages. Inthe particular example of FIG. 7, each laser diode package isincorporated into a device and needs to produce a predetermined targetemission wavelength.

At step 702, a first laser diode package is characterized. As part ofthe characterization, the operational bias conditions needed to producethe target emission wavelength by the laser diode are determined. Atstep 704, the drive current level for the laser diode that results inthe target emission wavelength is determined. Step 704 may includemeasuring the optical output of the laser diode under test conditions todetermine the drive current level that produces the target wavelength.At step 706, the drive current level as determined at step 704 is setfor the driver circuitry of the first laser diode package. Step 706 caninclude setting adjustable parameters within the diode driver circuitryto produce the desired drive current level.

At step 708, a second laser diode package is characterized and at step710, a drive current level for the second laser diode package thatresults in the target emission wavelength is determined. At step 712,the drive current level determined in step 710 is set for the drivercircuitry of the second laser diode package. Individual diode packageswill naturally contain variances that result in different drive currentlevels to produce the same emission wavelength. As such, the diodedriver circuitry for the first and second laser diode packages mayproduce different drive currents for their corresponding diode package.In a typical scenario, the operations in FIG. 7 would be repeated manymore times for additional diode packages.

By utilizing a low-level reference current for the junction temperaturemeasurements, standard circuitry and calculations can be utilized forthe different driver circuits to determine the junction temperature. Anindication of the actual drive current level need not be maintained withthe driver circuitry. This can be contrasted with techniques that maydrive the diode package at full power with the drive current and measurethe resulting voltage across the emissions diode to determine junctiontemperature. If a calculation is to be done at full drive current andpower to determine junction temperature, the drive current level needsto be known. If the drive current level for two packages is different,each driver circuit needs to maintain an indication of the drive currentlevel to be used in temperature calculations. In the presently disclosedembodiments however, a single low-level reference current can be usedfor multiple packages such that the driver circuitry for each can be thesame, even where the drive current levels are different.

FIGS. 8-10 depict an exemplary system, namely a tracking system usingdepth images, where the junction temperature calculations detailed abovemay be used. Tracking systems for human targets have a number of uses,including but not limited to gaming systems. FIGS. 8 and 9 illustrate asystem 10 with a user 18 playing a boxing game. Such a system 10 may beused to recognize, analyze, and/or track a human target such as the user18 or other objects within range of tracking system 10.

As shown in FIG. 8, tracking system 10 may include a computing system12. The computing system 12 may be a computer, a gaming system orconsole, or the like. According to one example, the computing system 12may include hardware components and/or software components such thatcomputing system 12 may be used to execute applications such as gamingapplications, non-gaming applications, or the like. In one embodiment,computing system 12 may include a processor such as a standardizedprocessor, a specialized processor, a microprocessor, or the like thatmay execute instructions stored on a processor readable storage devicefor performing the processes described herein.

As shown in FIG. 8, tracking system 10 may further include a capturedevice 20. The capture device 20 may be, for example, a camera that maybe used to visually monitor one or more users, such as the user 18, suchthat gestures and/or movements performed by the one or more users may becaptured, analyzed, and tracked to perform one or more controls oractions within the application and/or animate an avatar or on-screencharacter.

According to one embodiment, the tracking system 10 may be connected toan audio/visual device 16 such as a television, a monitor, ahigh-definition television (HDTV), or the like that may provide game orapplication visuals and/or audio to a user such as the user 18. Forexample, the computing system 12 may include a video adapter such as agraphics card and/or an audio adapter such as a sound card that mayprovide audio/visual signals associated with the game application,non-game application, or the like. The audio/visual device 16 mayreceive the audio/visual signals from the computing system 12 and maythen output the game or application visuals and/or audio associated withthe audio/visual signals to the user 18. According to one embodiment,the audio/visual device 16 may be connected to the computing system 12via, for example, an S-Video cable, a coaxial cable, an HDMI cable, aDVI cable, a VGA cable, component video cable, or the like.

As shown in FIGS. 8 and 9, the tracking system 10 may be used torecognize, analyze, and/or track a human target such as the user 18. Forexample, the user 18 may be tracked using the capture device 20 suchthat the gestures and/or movements of user 18 may be captured to animatean avatar or on-screen character and/or may be interpreted as controlsthat may be used to affect the application being executed by computerenvironment 12. Thus, according to one embodiment, the user 18 may movehis or her body to control the application and/or animate the avatar oron-screen character. Similarly, tracking system 10 may be used torecognize, analyze, and/or track persons who are watching user 18 playthe game so that movement by those persons watching user 18 play thegame will control movement avatars in the audience at the boxing gamedisplayed on audio/visual device 16.

In the example depicted in FIGS. 8 and 9, the application executing onthe computing system 12 may be a boxing game that the user 18 isplaying. The computing system 12 may use the audio/visual device 16 toprovide a visual representation of a boxing opponent 22 to the user 18.The computing system 12 may also use the audio/visual device 16 toprovide a visual representation of a user avatar 24 that the user 18 maycontrol with his or her movements. For example, as shown in FIG. 9, theuser 18 may throw a punch in physical space to cause the user avatar 24to throw a punch in game space. Thus, according to an exampleembodiment, the computer system 12 and the capture device 20 recognizeand analyze the punch of the user 18 in physical space such that thepunch may be interpreted as a game control of the user avatar 24 in gamespace and/or the motion of the punch may be used to animate the useravatar 24 in game space.

Other movements by the user 18 may also be interpreted as other controlsor actions and/or used to animate the user avatar, such as controls tobob, weave, shuffle, block, jab, or throw a variety of different powerpunches. Furthermore, some movements may be interpreted as controls thatmay correspond to actions other than controlling the user avatar 24. Forexample, in one embodiment, the user may use movements to end, pause, orsave a game, select a level, view high scores, communicate with afriend, etc. According to another embodiment, the user may use movementsto select the game or other application from a main user interface.Thus, in one example, a full range of motion of the user 18 may beavailable, used, and analyzed in any suitable manner to interact with anapplication.

In one example, the human target such as the user 18 may have an object.In such embodiments, the user of an electronic game may be holding theobject such that the motions of the user and the object may be used toadjust and/or control parameters of the game. For example, the motion ofa user holding a racket may be tracked and utilized for controlling anon-screen racket in an electronic sports game. In another exampleembodiment, the motion of a user holding an object may be tracked andutilized for controlling an on-screen weapon in an electronic combatgame. Objects not held by the user can also be tracked, such as objectsthrown, pushed or rolled by the user (or a different user) as well asself propelled objects. In addition to boxing, other games can also beimplemented.

According to other examples, the tracking system 10 may further be usedto interpret target movements as operating system and/or applicationcontrols that are outside the realm of games. For example, virtually anycontrollable aspect of an operating system and/or application may becontrolled by movements of the target such as the user 18.

FIG. 10 illustrates an example of the capture device 20 that may be usedin the tracking system 10. According to an example, the capture device20 may be configured to capture video with depth information including adepth image that may include depth values via any suitable techniqueincluding, for example, time-of-flight, structured light, stereo image,or the like. According to one embodiment, the capture device 20 mayorganize the depth information into “Z layers,” or layers that may beperpendicular to a Z axis extending from the depth camera along its lineof sight.

As shown in FIG. 10, the capture device 20 may include an image cameracomponent 23. According to an example, the image camera component 23 maybe a depth camera that may capture a depth image of a scene. The depthimage may include a two-dimensional (2-D) pixel area of the capturedscene where each pixel in the 2-D pixel area may represent a depth valuesuch as a distance in, for example, centimeters, millimeters, or thelike of an object in the captured scene from the camera.

As shown in FIG. 10, according to an example, the image camera component23 may include an infra-red (IR) light component 25, a three-dimensional(3-D) camera 26, and an RGB camera 28 that may be used to capture thedepth image of a scene. For example, in time-of-flight analysis, the IRlight component 25 of the capture device 20 may emit infrared light at apredetermined wavelength onto the scene to illuminate the scene and oneor more targets and objects in the scene. Sensors (not shown) are thenused to detect the backscattered light from the surface of one or moretargets and objects in the scene using, for example, the 3-D camera 26and/or the RGB camera 28. In some embodiments, pulsed infrared light maybe used such that the time between an outgoing light pulse and acorresponding incoming light pulse may be measured and used to determinea physical distance from the capture device 20 to a particular locationon the targets or objects in the scene. Additionally, in other examples,the phase of the outgoing light wave may be compared to the phase of theincoming light wave to determine a phase shift. The phase shift may thenbe used to determine a physical distance from the capture device to aparticular location on the targets or objects.

According to another example, time-of-flight analysis may be used toindirectly determine a physical distance from the capture device 20 to aparticular location on the targets or objects by analyzing the intensityof the reflected beam of light over time via various techniquesincluding, for example, shuttered light pulse imaging.

In another example, the capture device 20 may use a structured light tocapture depth information. In such an analysis, patterned light (i.e.,light displayed as a known pattern such as grid pattern, a stripepattern, or different pattern) may be projected onto the scene via, forexample, the IR light component 25. Upon striking the surface of one ormore targets or objects in the scene, the pattern may become deformed inresponse. Such a deformation of the pattern may be captured by, forexample, the 3-D camera 26 and/or the RGB camera 28 (and/or othersensor) and may then be analyzed to determine a physical distance fromthe capture device to a particular location on the targets or objects.In some implementations, the IR Light component 25 is displaced from thecameras 24 and 26 so triangulation can be used to determined distancefrom cameras 26 and 28. In some implementations, the capture device 20will include a dedicated IR sensor to sense the IR light, or a sensorwith an IR filter.

In one embodiment, the infra-red (IR) light component 25 may include atleast one laser diode package for generating and emitting the infra-redlight. In tracking applications that utilize such IR light components,the operational precision of the laser diode may be of increasedimportance as compared with other applications. In such cases, theemissions of the laser diode may need to be held at a constantwavelength. Because the wavelength of the diode's emissions will varywith temperature, regulating the temperature of the diode package takeson significance. Accordingly, in one embodiment, a laser diode IR lightcomponent 25 is maintained at a target operating temperature byutilizing the junction temperature calculations detailed above so thatthe emission wavelength of the diode is held constant or substantiallyconstant. Further, because a manufacturer may build multiple trackingsystems, with individual diode packages having unique operatingcharacteristics, the drive current levels for different devices may varyas noted above. Accordingly, the use of a low-level reference currentenables multiple devices to be manufactured with different drive currentlevels, while enabling temperature calculations to be done usingstandard drive circuitry. In alternate techniques whereby a full drivecurrent is used to determine junction temperature, individual parametersneed to be maintained including the drive current level so that thejunction temperature can be calculated.

The capture device 20 may further include a microphone 30. Themicrophone 30 may include a transducer or sensor that may receive andconvert sound into an electrical signal. According to one embodiment,the microphone 30 may be used to reduce feedback between the capturedevice 20 and the computing system 12 in the target recognition,analysis, and tracking system 10. Additionally, the microphone 30 may beused to receive audio signals that may also be provided by to computingsystem 12.

In an example, the capture device 20 may further include a processor 32that may be in communication with the image camera component 23. Theprocessor 32 may include a standardized processor, a specializedprocessor, a microprocessor, or the like that may execute instructionsincluding, for example, instructions for receiving a depth image,generating the appropriate data format (e.g., frame) and transmittingthe data to computing system 12.

The capture device 20 may further include a memory component 34 that maystore the instructions that are executed by processor 32, images orframes of images captured by the 3-D camera and/or RGB camera, or anyother suitable information, images, or the like. According to an exampleembodiment, the memory component 34 may include random access memory(RAM), read only memory (ROM), cache, flash memory, a hard disk, or anyother suitable storage component. As shown in FIG. 10, in oneembodiment, memory component 34 may be a separate component incommunication with the image capture component 23 and the processor 32.According to another embodiment, the memory component 34 may beintegrated into processor 32 and/or the image capture component 23.

As shown in FIG. 10, capture device 20 may be in communication with thecomputing system 12 via a communication link 36. The communication link36 may be a wired connection including, for example, a USB connection, aFirewire connection, an Ethernet cable connection, or the like and/or awireless connection such as a wireless 802.11b, g, a, or n connection.According to one embodiment, the computing system 12 may provide a clockto the capture device 20 that may be used to determine when to capture,for example, a scene via the communication link 36. Additionally, thecapture device 20 provides the depth information and visual (e.g., RGB)images captured by, for example, the 3-D camera 26 and/or the RGB camera28 to the computing system 12 via the communication link 36. In oneembodiment, the depth images and visual images are transmitted at 30frames per second. The computing system 12 may then use the model, depthinformation, and captured images to, for example, control an applicationsuch as a game or word processor and/or animate an avatar or on-screencharacter.

Computing system 12 includes depth image processing and skeletaltracking module 50, which uses the depth images to track one or morepersons detectable by the depth camera. Depth image processing andskeletal tracking module 50 provides the tracking information toapplication 52, which can be a video game, productivity application,communications application or other software application, etc. The audiodata and visual image data is also provided to application 52 and depthimage processing and skeletal tracking module 50. Application 52provides the tracking information, audio data and visual image data torecognizer engine 54. In another embodiment, recognizer engine 54receives the tracking information directly from depth image processingand skeletal tracking module 50 and receives the audio data and visualimage data directly from capture device 20. Recognizer engine 54 isassociated with a collection of filters 60, 62, 64, . . . , 66, eachcomprising information concerning a gesture or other action or eventthat may be performed by any person or object detectable by capturedevice 20. For example, the data from capture device 20 may be processedby the filters 60, 62, 64, . . . , 66 to identify when a user or groupof users has performed one or more gestures or other actions. Thosegestures may be associated with various controls, objects or conditionsof application 52. Thus, the computing environment 12 may use therecognizer engine 54, with the filters, to interpret movements.

Capture device 20 of FIG. 10 provides RGB images (or visual images inother formats or color spaces) and depth images to computing system 12.A depth image may be a plurality of observed pixels where each observedpixel has an observed depth value. For example, the depth image mayinclude a two-dimensional (2-D) pixel area of the captured scene whereeach pixel in the 2-D pixel area may have a depth value such as distanceof an object in the captured scene from the capture device.

The system will use the RGB images and depth images to track a user'smovements. For example, the system will track a skeleton of a personusing a depth images. There are many methods that can be used to trackthe skeleton of a person using depth images. One suitable example oftracking a skeleton using depth images is provided in U.S. patentapplication Ser. No. 12/603,437, “Pose Tracking Pipeline,” filed on Oct.21, 2009. (hereinafter referred to as the '437 Application),incorporated herein by reference in its entirety. The process of the'437 Application includes acquiring a depth image, down sampling thedata, removing and/or smoothing high variance noisy data, identifyingand removing the background, and assigning each of the foreground pixelsto different parts of the body. Based on those steps, the system willfit a model with the data and create a skeleton. The skeleton willinclude a set of joints and connections between the joints. Suitabletracking technology is also disclosed in U.S. patent application Ser.No. 12/475,308, “Device for Identifying and Tracking Multiple HumansOver Time,” filed on May 29, 2009, incorporated herein by reference inits entirety; U.S. patent application Ser. No. 12/696,282, “Visual BasedIdentity Tracking,” filed on Jan. 29, 2010, incorporated herein byreference in its entirety; U.S. patent application Ser. No. 12/641,788,“Motion Detection Using Depth Images,” filed on Dec. 18, 2009,incorporated herein by reference in its entirety; and U.S. patentapplication Ser. No. 12/575,388, “Human Tracking System,” filed on Oct.7, 2009, incorporated herein by reference in its entirety.

Gesture recognizer engine 54 (of computing system 12 depicted in FIG. 2)is associated with multiple filters 60, 62, 64, . . . , 66 to identify agesture or action. A filter comprises information defining a gesture,action or condition along with parameters, or metadata, for thatgesture, action or condition. For instance, a throw, which comprisesmotion of one of the hands from behind the rear of the body to past thefront of the body, may be implemented as a gesture comprisinginformation representing the movement of one of the hands of the userfrom behind the rear of the body to past the front of the body, as thatmovement would be captured by the depth camera. Parameters may then beset for that gesture. Where the gesture is a throw, a parameter may be athreshold velocity that the hand has to reach, a distance the hand musttravel (either absolute, or relative to the size of the user as awhole), and a confidence rating by the recognizer engine that thegesture occurred. These parameters for the gesture may vary betweenapplications, between contexts of a single application, or within onecontext of one application over time. In one embodiment, a filter has anumber of inputs and a number of outputs.

Filters may be modular or interchangeable so that a first filter may bereplaced with a second filter that has the same number and types ofinputs and outputs as the first filter without altering any other aspectof the recognizer engine architecture. For instance, there may be afirst filter for driving that takes as input skeletal data and outputs aconfidence that the gesture associated with the filter is occurring andan angle of steering. Where one wishes to substitute this first drivingfilter with a second driving filter—perhaps because the second drivingfilter is more efficient and requires fewer processing resources—one maydo so by simply replacing the first filter with the second filter solong as the second filter has those same inputs and outputs—one input ofskeletal data type, and two outputs of confidence type and angle type.

A filter need not have a parameter. For instance, a “user height” filterthat returns the user's height may not allow for any parameters that maybe tuned. An alternate “user height” filter may have tunableparameters—such as to whether to account for a user's footwear,hairstyle, headwear and posture in determining the user's height.

Inputs to a filter may comprise things such as joint data about a user'sjoint position, like angles formed by the bones that meet at the joint,RGB color data from the scene, and the rate of change of an aspect ofthe user. Outputs from a filter may comprise things such as theconfidence that a given gesture is being made, the speed at which agesture motion is made, and a time at which a gesture motion is made.

Gesture recognizer engine 54 provides functionality to the filters. Inone embodiment, the functionality that the recognizer engine 54implements includes an input-over-time archive that tracks recognizedgestures and other input, a Hidden Markov Model implementation (wherethe modeled system is assumed to be a Markov process—one where a presentstate encapsulates any past state information necessary to determine afuture state, so no other past state information must be maintained forthis purpose—with unknown parameters, and hidden parameters aredetermined from the observable data), as well as other functionality tosolve particular instances of gesture recognition.

Filters 60, 62, 64, . . . , 66 are loaded and implemented on top ofrecognizer engine 54 and can utilize services provided by recognizerengine 54 to all filters 60, 62, 64, . . . 66. In one embodiment,recognizer engine 54 receives data to determine whether it meets therequirements of any filter 60, 62, 64, . . . , 66. Since these providedservices, such as parsing the input, are provided once by recognizerengine 54, rather than by each filter 60, 62, 64, . . . , 66, such aservice need only be processed once in a period of time as opposed toonce per filter for that period so the processing to determine gesturesis reduced.

Application 52 may use the filters 60, 62, 64, . . . , 66 provided bythe recognizer engine 54, or it may provide its own filters which plugsinto recognizer engine 54. In one embodiment, all filters have a commoninterface to enable this plug-in characteristic. Further, all filtersmay utilize parameters, so a single gesture tool below may be used todebug and tune the entire filter system.

More information about recognizer engine 54 can be found in U.S. patentapplication Ser. No. 12/422,661, “Gesture Recognizer SystemArchitecture,” filed on Apr. 13, 2009, incorporated herein by referencein its entirety. More information about recognizing gestures can befound in U.S. patent application Ser. No. 12/391,150, “StandardGestures,” filed on Feb. 23,2009; and U.S. patent application Ser. No.12/474,655, “Gesture Tool” filed on May 29, 2009. Both of which areincorporated by reference herein in their entirety.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims. It is intended that the scopeof the invention be defined by the claims appended hereto.

We claim:
 1. A method of operating an emission diode for junctiontemperature determination, comprising: characterizing the emission diodeby measuring an operating temperature of the emission diode whenproducing emissions at a predetermined wavelength; setting a targetoperating temperature equal to the measured operating temperature;providing a drive current to the emission diode at a level sufficient toenable emissions by the emission diode; removing the drive current fromthe emission diode; providing a reference current to the emission diodeat a level less than the drive current level after removing the drivecurrent from the emission diode; determining a forward voltage level ofthe emission diode under application of the reference current; andcontrolling an operating temperature of the emission diode based on theforward voltage level and the target operating temperature.
 2. A methodaccording to claim 1, wherein controlling the operating temperature ofthe emission diode includes: comparing the forward voltage level of theemission diode to a reference voltage level corresponding to the targetoperating temperature for the emission diode; if the forward voltagelevel of the emission diode is higher than the reference voltage level,heating the emission diode; and if the forward voltage level of theemission diode is lower than the reference voltage level, cooling theemission diode.
 3. A method according to claim 1, wherein controllingthe operating temperature of the emission diode includes: determining ajunction temperature of the emission diode based on the forward voltagelevel and the reference current level; comparing the junctiontemperature of the emission diode to the target operating temperaturefor the emission diode; heating the emission diode if the junctiontemperature is lower than the target operating temperature; and coolingthe emission diode if the junction temperature is higher than the targetoperating temperature.
 4. A method according to claim 1, whereindetermining a junction temperature of the emission diode includes:accessing a lookup table; and determining the junction temperature froman entry in the lookup table containing the measured forward voltagelevel.
 5. A method according to claim 1, wherein characterizing theemission diode includes: measuring the forward voltage of the emissionsdiode at a plurality of known temperatures; and generating the lookuptable with entries containing the measured forward voltages and thecorresponding known temperature.
 6. A method according to claim 1,wherein the emission diode is in a package with a photodiode, the methodfurther comprising: providing the reference current to the photodiode;and determining a forward voltage level of the photodiode underapplication of the reference current; wherein controlling the operatingtemperature of the emission diode is based on the forward voltage levelof the emission diode and the forward voltage level of the photodiode.7. A method according to claim 1, wherein the drive current is a firstdrive current, the method further comprising: characterizing theemission diode; determining that the first drive current causesemissions by the emission diode at a predetermined wavelength; andsetting the level of the first drive current as an output of a firstdiode driver circuit for the emission diode.
 8. A method according toclaim 7, wherein the emission diode is a first emission diode, themethod further comprising: characterizing a second emission diode;determining that a second drive current causes emissions by the secondemission diode at the predetermined wavelength, a level of the seconddrive current being different than the level of the first drive current;setting the level of the second drive current as an output of a seconddiode driver circuit for the second emission diode; providing the seconddrive current to the second emission diode to cause emissions by thesecond emission diode at the predetermined wavelength; providing thereference current to the second emission diode after removing the seconddrive current from the second emission diode; determining a forwardvoltage level of the second emission diode under application of thereference current; and controlling an operating temperature of thesecond emission diode based on the forward voltage level of the secondemission diode.
 9. A method according to claim 1, further comprising:applying the drive current to the emission diode after providing thereference current.
 10. A method according to claim 1, wherein thereference current is not sufficient to cause emissions by the emissiondiode.
 11. A method according to claim 1, wherein the emission diode isa laser diode.
 12. A method according to claim 1, wherein the emissiondiode is a light-emitting diode.
 13. An emission diode driver,comprising: at least one current source, the at least one current sourcesupplies a drive current for an emission diode coupled to the emissiondiode driver and a reference current for the emission diode; at leastone voltage measurement circuit, the at least one voltage measurementcircuit measures a forward voltage of the emission diode while thereference current is applied to the emission diode; and at least onecontrol circuit that controls an operating temperature of a diodepackage based on the measured forward voltage of the emission diode anda target operating temperature that is equal to a characterizedoperating temperature of the emission diode when producing emissions ata predetermined wavelength.
 14. An emission diode driver according toclaim 13, wherein: the at least one control circuit includes atemperature controller; the at least one control circuit compares themeasured forward voltage of the emission diode to a reference voltagelevel corresponding to the target operating temperature for the diodepackage; and the temperature controller heats the diode package if themeasured forward voltage of the photodiode is higher than the referencevoltage level and cools the diode package if the measured forwardvoltage of the photodiode is lower than the reference voltage level. 15.An emission diode driver according to claim 13, wherein: the at leastone control circuit includes a temperature controller and a lookuptable; the at least one control circuit determines a junctiontemperature of the emission diode from an entry in the lookup tablecontaining the measured forward voltage and compares the junctiontemperature to the target operating temperature for the diode package;and the temperature controller heats the diode package if the junctiontemperature is lower than the target operating temperature and cools thediode package if the junction temperature is higher than the targetoperating temperature.
 16. An emission diode driver according to claim13, wherein: the emission diode is in a package with a photodiode: theat least one current source removes the drive current from the emissiondiode while supplying the reference current to the emission diode; theat least one current source supplies the reference current to thephotodiode after removing the drive current; the at least one voltagemeasurement circuit measures a forward voltage of the photodiode whilethe reference current is applied to the photodiode; and the at least onecontrol circuit controls the operating temperature of the diode packagebased on the measured forward voltage of the photodiode and the measuredforward voltage of the emission diode.
 17. A method of operating anemission diode in a diode package with a photodiode, comprising:characterizing the emission diode package by measuring an operatingtemperature of the emission diode when producing emissions at apredetermined wavelength; setting a reference forward voltage for thephotodiode based on the measured operating temperature of the emissiondiode; providing a drive current to the emission diode at a levelsufficient to enable emissions by the emission diode; removing the drivecurrent from the emission diode; providing a reference current to thephotodiode at a level less than the drive current level after removingthe drive current from the emission diode; determining a forward voltagelevel of the photodiode under application of the reference current; andcontrolling an operating temperature of the emission diode based oncomparing the forward voltage level of the photodiode to the referenceforward voltage.
 18. A method according to claim 17, wherein setting thereference forward voltage for the photodiode based on the measuredoperating temperature comprises: measuring a forward voltage of thephotodiode when the emission diode is at the measured operatingtemperature for producing emissions at the predetermined wavelength. 19.A method according to claim 18, wherein characterizing the emissiondiode package includes: measuring the forward voltage of the photodiodeat a plurality of known temperatures for the emission diode; andgenerating the lookup table with entries containing the measured forwardvoltages of the photodiode and the corresponding known temperature.